Pacing to the left atrium is important for successful bi-atrial pacing. The pacing to the left atrium is usually accomplished by placing a pacing lead into the coronary sinus, which is a venous structure accessible through the right atrium of the heart and serves to drain the coronary veins. The coronary sinus is a curved, generally tubular structure typically having a lesser radius of curvature (the inner side of the curved tube) and a greater radius of curvature (the outer side of the curved tube). The coronary sinus is generally wider at its ostium and tapers inwardly away from the ostium towards the distal portions of the coronary sinus. The ostium of the coronary sinus is located at the juncture of the right atrium and the right ventricle.
To pace the left atrium, a pacing lead can be positioned so that an electrode contacts the wall of the coronary sinus closest to the left atrium. Because the coronary sinus is in electrical contact with the left atrium, by pacing the coronary sinus at this position, one can also pace the left atrium. The pacing lead generally is advanced to the ostium of the coronary sinus through the right atrium portion of the right heart. For effective pacing, electrodes on the pacing lead should be in constant electrical conductive contact with the wall of the coronary sinus, preferably the left atrial side of the coronary sinus. Conductivity is preferably sufficient so as to enable a pacing voltage of 3 volts or less.
To accomplish constant wall contact, different pacing lead configurations have been used to assist in the placement and retention of the pacing lead in the desired position. These prior leads, however, all have certain drawbacks making them not entirely satisfactory. For example, leads have been used in which a body of the lead is pre-formed to have a sinusoidal or helical configuration enabling the lead to expand into contact with the walls of the coronary sinus and retain the lead. Examples of such pre-formed coronary sinus leads are disclosed in U.S. Pat. No. 5,423,865 to Bowald et al. and U,S. Pat. No. 5,476,498 to Ayers. Such shapes, however, if not carefully sized to the diameter of the coronary sinus do not necessarily bias the electrodes against the wall of the coronary sinus with sufficient force to ensure good electrical connectivity resulting in instability and high pacing thresholds. Moreover, with helical shape leads, the electrodes may be difficult to properly position relative to the coronary sinus wall for optimal contact and thresholds.
Referring to FIGS. 1a-1f, specific examples of pre-formed coronary sinus leads are depicted and described in U.S. Pat. No. 6,321,123 to Morris et al, which is incorporated herein by reference. Pacing leads 20 according to Morris et al. generally include a proximal lead portion 21 with a first curved portion 22, a second curved portion 24, a tip electrode 26, and additional electrodes 28 longitudinally disposed relative to proximal lead portion 21. A sheath and straightening stylet or guidewire is used to insert the lead 20 tip first into a coronary sinus. Once the sheath and lead 20 are within the coronary sinus, the sheath is removed and the lead 20 resiliently flexes toward its pre-formed shape. The pre-formed “J” in the lead 20 can cause the tip 26 to be pressed up against the wall of the coronary sinus as depicted in FIGS. 1c and 1d, but only if the width of the pre-formed pacing lead 20 is greater than that of the coronary sinus. The tip and other electrodes, however, may not be in an optimal geometric relationship with the wall of the coronary sinus, since the lead cannot fully assume its preformed shape. Moreover, the biasing force exerted by the lead has both a laterally directed component, annotated as F1 in the figures, and a longitudinally directed component, annotated as F2 in the figures. The longitudinally directed component of the biasing force tends to urge the lead longitudinally backward in the coronary sinus toward the ostium, thereby reducing lead stability.
When the coronary sinus is wider than the pacing lead 20, the lead may assume a loop shape longitudinally disposed relative to proximal lead portion 21 as depicted in FIGS. 1b, 1e and 1f. In this case, the longitudinally directed component of the biasing force F2 is generally relatively less due to the contact of the lead with the coronary sinus wall laterally opposite the tip 26, but the resilience of the lead may provide little or no laterally directed biasing force F1 for good electrical conductivity, and in some cases the tip electrode can lose contact with the wall of the coronary sinus altogether. This can lead to higher voltage requirements, greater instability and higher pacing and sensing thresholds.
Referring to FIG. 2, there are also other pacing leads 30 that can be used to pace the left atrium through the coronary sinus, such as the Medtronic Attain® Bipolar OTW Lead Model No. 4194 and leads as disclosed in U.S. Pat. No. 5,683,445 to Swoyer, both of which are incorporated herein by reference. The pacing lead 30 generally includes a first curved portion 32, a second curved portion 34, and a tip electrode 36. As depicted in FIG. 2, the angle 31 at the first curve 32 is greater than ninety degrees. Again, a sheath and stylet are used to insert the lead 30 tip first into a coronary sinus. Once the sheath and lead 30 are within the coronary sinus, the sheath and stylet are removed and the lead 30 takes its pre-formed shape, enabling the tip electrode 36 to contact the walls of the coronary sinus if the diameter of the coronary sinus is less than the width of the pre-formed shape of the lead. Again however, using these leads to pace the coronary sinus presents the same problems inherent with the leads according to Morris et al. as described above.
The present inventor has recognized that prior art leads and fixation methods provide a success rate of 60% or less when used to pace the left atrium via the coronary sinus. Hence, there is still a need for a lead and fixation method assuring stable pacing of the left atrium through the coronary sinus. Because the general problems discussed above have not been addressed by conventional pacing leads, there is a current need for pacing leads addressing the problems and deficiencies inherent with conventional designs.